The present invention relates to a distance measuring system for measuring a distance using light wave, and in particular, to a distance measuring system, by which the distance can be measured both when a reflection prism is mounted on an object to be measured and when it is not mounted.
A distance measuring system using light wave is a system, which can measure a distance from a measuring position to an object to be measured by projecting a range-finding light beam toward the object to be measured and by receiving a reflection light beam reflected by the object to be measured. In general, it is divided to two types: a distance measuring system which requires a reflection prism (a corner cube) on the object to be measured, and a non-prism distance measuring system which can measure the distance without a reflection prism. However, the arrangement and the process from light emission to photodetection is approximately the same in these two types of the system.
Referring to FIG. 6, description will be given below on an essential portion of the distance measuring system.
The distance measuring system comprises an optical unit 1 for projecting and receiving a range-finding light beam and an electrical unit (not shown) for converting the received range-finding light beam to an electric signal and for calculating the distance. The optical unit 1 for projecting and receiving the range-finding light beam comprises a light emitting unit for emitting a range-finding light beam 3' from a light source 2, a projecting optical system 5 for projecting the range-finding light beam 3 from the light emitting unit toward an object to be measured 4, a photodetection optical system 6 for guiding a reflection light beam 3' from the object to be measured 4, and a photodetection unit 7 for receiving the reflection light beam 3' guided by the photodetection optical system 6.
When the range-finding light beam (a pulsed laser beam or a modulated laser beam) 3 is emitted from the light source 2 (such as a semiconductor laser) and it is deflected by a projection side half-mirror 10, a rotary shading disk 11, and a reflection mirror 12, it is directed toward an objective lens 13, which serves as a projection optical system. After being turned to the approximately parallel beam by the objective lens 13, the range-finding light beam 3 is projected toward the object to be measured 4 (the reflection prism) via the objective lens 13. After being reflected by the object to be measured 4, the reflection light beam 3' is directed toward the objective lens 13 again. Being directed toward the objective lens 13, the reflection light beam 3' is focused by the objective lens 13 and is deflected by the reflection mirror 12. After passing through a light quantity attenuation filter 14 and a photodetection side half-mirror 15, the beam forms an image on the photodetection unit 7 and is received. The light quantity attenuation filter 14 has a range-finding laser beam attenuation filter on the outer periphery and a reference laser beam attenuation filter on the inner portion, and the density is gradually changed in the circumferential direction.
After being emitted from the light source 2, the laser beam is divided and reflected by the projection side half-mirror 10, and it is deflected by a reflection mirror 17 as a reference light beam 16. After passing through the rotary shading disk 11, a pair of relay lenses 18 and 19, and the light quantity attenuation filter 14, it is reflected by a deflection mirror 21 and enters the photodetection unit 7.
The rotary shading disk 11 is rotated and driven by a motor 23 and selectively shades the light beam so that the reference light beam 16 and the range-finding light beam 3 as separated by the projection side half-mirror 10 alternately enter the photodetection unit 7. The reference light beam is used to correct an internal error in the distance measuring system.
After being reflected by the object to be measured 4, the reflection light beam 3' reaches and is received by the photodetection unit 7 via the objective lens 13, the reflection mirror 12 and the light quantity attenuation filter 14. In order to avoid the influence of the changes the photodetection light quantity to the photodetection characteristics, the light quantity attenuation filter 14 is rotated by a motor 24, and transmitting positions of the range-finding light beam 3 and the reflection light beam 3' are changed with respect to the light quantity attenuation filter 14. As a result, the light quantity is adjusted so that the reflection light beam 3' entering the photodetection unit 7 has constant photodetection light quantity.
On the photodetection unit 7, the reference light beam 16 with its photodetection light quantity adjusted by the light quantity attenuation filter 14 is received. The electrical unit (not shown) converts the range-finding light beam 3 and the reference light beam 16 to electric signals, and the distance to the object to be measured 4 is calculated from these two electric signals.
FIG. 7(A) shows the condition of optical paths of the range-finding light beam 3 and the reflection light beam 3' when the object to be measured 4 is a prism (or when a prism is mounted on the object to be measured).
The reflection light beam received by the photodetection unit 7 is basically the same as the projected range-finding light beam, and it is separated to the projection optical system 5 and the photodetection optical system 6 by the reflection mirror 12. In case of a type of the system generally used where the single objective lens 13 is commonly used by the projection optical system 5 and the photodetection optical system 6, the objective lens 13 is separated into upper and lower parts or into left and right parts, or into central part and outer peripheral part for the projection optical system and the photodetection optical system. This arrangement is the same in an electronic theodolite with a built-in distance measuring system. When a distance to a corner cube positioned at short distance is measured by the distance measuring system as described above, a deviation occurs between the projected range-finding light beam 3 and the reflection light beam 3' as shown in FIG. 7(A). For this reason, even when the projection optical system 5 including the light source 2 and the photodetection optical system 6 including the photodetection unit 7 may not be positioned on the same axis, the distance can be measured.
On the other hand, FIG. 7(B) shows a case where the object to be measured 4' is a low-retroreflection sheet (made of very small spherical glass pieces) or a natural object (having a certain reflection characteristic).
When the reflection sheet or the natural object is used as the object to be measured 4', the range-finding light beam 3 projected to the object to be measured 4' is turned to the reflection light beam 3', which is diffused around a projected position J1. In this respect, the diffused reflection light beam 3' from the object to be measured 4' entering the photodetection optical system 6 is tilted with respect to the optical axis of the objective lens 13. Thus, it is deviated from the photodetection unit 7, and it is difficult to form an image. When an image is formed at a position away from the photodetection unit 7, the substantial light quantity received on the photodetection unit 7 is extremely decreased, and it is difficult to measure the distance. This causes more problems in case of the non-prism distance measurement, in which the reflection sheet or the natural object is used as the object to be measured 4' and there is no discriminative selection on the object. When the non-prism distance measurement is performed for long distance measurement, no problem occurs because the reflection light beam 3' enters the photodetection optical system 6 almost in parallel. However, the reflection light quantity itself is substantially decreased, and the distance cannot be measured. Therefore, the non-prism distance measurement is adopted for the measurement of the short distance. In case of the short distance measurement, the tilting of the reflection light beam as described above becomes an issue.
For this reason, the countermeasures as shown in FIG. 8(A) and FIG. 8(B) have been taken in the past.
Specifically, a prism 26 for compensation or a compensating lens 27 are provided on the optical paths between the objective lens 13 and the object to be measured 4'. These compensating optical means are added to the optical system in case of the non-prism distance measurement, and a part of the reflection light beam diffused by the object to be measured 4' is directed to the photodetection unit 7 to form an image.
However, the mounting of the above prism 26 or the compensating lens 27 for each short distance measurement means lower working efficiency. Also, the components for compensation must be separately provided, and this leads to the problem of the increased manufacturing cost. Further, there is also a problem in that the image of collimation is turned to dual images.